Added Streaming Silent (Blind) Mode

### Describe the feature or problem you'd like to solve

Keep users focused by making their stream invisible

### Proposed solution

Stream history is not for review in most cases.

Command-confirmation has become more important.

Maintain decision-making quality by masking distracting llm stream

### Example prompts or workflows

1. Stream on/off can be controlled with slash commands.
2. After the command, all streams are hidden and an overlay screen is displayed.
3. The overlay screen has a Copilot logo in the center and hours, minutes, seconds at the bottom.
4. A heartbeat animation effect may be needed.
5. Add a shortcut key to dismiss the overlay while streaming.

### Additional context